As mobile and stationary radio communication is promoted to broadbandaization in the future, a large-sized base station apparatus, or short duration and heat radiation of a mobile terminal's battery according to an increase in power consumption of a power amplifier become a big problem. For this reason, a power amplifier is required to be capable of amplifying a high speed signal having a wide bandwidth with low distortion and high efficiency.
Methods of realizing high-efficiency modulation and amplification include a Doherty amplification scheme, an ET (Envelope tracking) scheme, an EER (Envelope Elimination Restoration) scheme and so on.
Meanwhile, to increase the transmission capacity of a transmission signal, a M-ary modulation scheme such as QAM (Quadrature Amplitude Modulation) is adopted.
Further, to increase frequency spectrum efficiency, an OFDM scheme (Orthogonal Frequency Division Multiplexing), whereby multicarrier signals are mapped densely in the frequency domain, is adopted.
Furthermore, large capacity transmission is possible by MIMO (Multi Input Multi Output) transmission scheme using a plurality of antennas.
FIG. 1 shows a configuration of a transmitting apparatus adopting a 2×2 MIMO transmission scheme. As shown in the figure, transmitting apparatus 10 has two transmission systems each configured with power amplifier 1, antenna 2 and signal generator 3. Signal generator 3-1 receives transmission data 1 and performs radio processing including up-conversion on transmission data 1, to generate a radio signal sequence. Power amplifier 1-1 receives the radio signal sequence generated in signal generator 3-1 and amplifies this radio signal sequence. The amplified radio signal sequence is transmitted via antenna 2-1. The same processing is performed on transmission data sequence 2 in the other transmission system.
Incidentally, by employing the above M-ary modulation scheme and OFDM scheme, an envelop signal of a radio transmission signal sequence tends to fluctuate significantly. That is, the signal tends to be a wide dynamic range signal. This fluctuation width of this envelop signal is represented by an indicator referred to as “PAPR” (Peak-to-Average-Power-Ratio). Generally, when PAPR is high, it is necessary to increase the back-off of an amplifier, and therefore the efficiency of an amplifier decreases. For this reason, it is important to maintain the efficiency of an amplifier high even in a wide dynamic range signal.
FIG. 2 is a block diagram showing the configuration of a transmitting apparatus adopting the high-efficiency Doherty amplification scheme disclosed in Patent Document 1.
In this transmitting apparatus, a plurality of peak amplifiers 16 are disposed in parallel with one carrier amplifier 14. Splitter 17 splits a signal received as input into a plurality of signals. Further, splitter 17 changes the signal level ratio of a plurality of split signals. By this means, it is possible to set up different input levels in which a plurality of peak amplifiers 16 turn on between a plurality of peak amplifiers 16. In this way, different peak amplifiers are allowed to operate in a saturation mode according to signal levels, and this makes it possible to operate amplifiers with high efficiency even in a wide dynamic range.
Patent Document 1: Japanese Translation of a PCT Application Laid-Open No. 2005-525727